Polyamide yarns are frequently employed in industrial yarn and fabric applications requiring high strength. In order to develop maximum strength nylon yarns are manufactured by a spinning and drawing process that causes molecular alignment. The higher degree of orientation that is achieved, the greater is the tenacity and the lower is the available yarn elongation. A fundamental aspect of the production of fabrics using high tenacity yarns made with polyamides relates to the inherent shrinkage of the yarn. Due to the fact that the polymer undergoes a high degree of molecular alignment in the spinning and drawing process, such yarn has a natural tendency to contract. The rate and degree of contraction is a function of the degree of drawing (where more drawing leads to greater degree of contraction), the temperature to which the yarn is heated, and the time for which the yarn is held at temperature. Hence, it is normal to wash fabric in hot water and then dry in hot air in order to promote shrinkage and cause to fabric to become dimensionally stable. The degree of contraction of the fiber affects the efficiency of production of fabrics by virtue of a decrease in utilization of as-woven fabric as the fabric shrinkage encountered during post-weaving processing increases.
Known processes for making fully-drawn nylon yarns include the steps of extruding molten polymer through a spinneret to form filaments, quenching the molten filaments, coalescing the filaments to form a multifilament yarn and then drawing the yarn to increase molecular orientation, reduce available elongation and develop increased tenacity. Drawing is achieved by advancing the as-spun yarn from a feed roll to a draw roll, wherein the draw roll is rotating at a higher speed than the feed roll. The greater the extent of the drawing, the higher will be the yarn shrinkage. A process of this type, in which the spinning and drawing steps are integrated into a continuous manufacturing process, is referred to as a “spin-draw” process.
It is possible to produce very low shrinkage polyamide yarns using slow “two stage” processes, where the drawing is done in a separate step after the as-spun yarn has been wound and, therefore, the drawing and relaxing stages are decoupled from spinning. However, the product is found to be too crystalline prior to drawing to allow for very high draw levels without experiencing yarn breaks. Thus, the “two stage” process is not suitable for high production rate manufacture of very high tenacity yarns above about 80 cN/tex.
Highly drawn, high shrinkage yarns produced by the spin-draw process can cause subsequent processing problems due to the tension induced in the yarns by the drawing step. If not relieved, the tension may be high enough to cause the cardboard tube core on which the yarn package is wound to deform. Additionally, the low elongation resulting from the high degree of drawing can lead to an unacceptable number of yarn breaks. This problem increases in severity with the high threadline speeds that are necessary for economic high speed production.
In order to alleviate the problems of package deformation and threadline breakage, it is known to introduce a relaxation step following drawing in order to reduce the yarn tension, usually while heating, prior to wind-up. One such process has been disclosed in U.S. Pat. No. 5,750,215 to Jaegge et al., the teachings of which are incorporated by reference. U.S. Pat. No. 5,750,215 employs a relaxation step in order to produce yarn package comprising nylon 6,6 yarn, such yarn characterized by an elongation of about 22% to about 60%, a boil-off shrinkage of about 3% to about 10%, a tenacity of about 3 to about 7 grams per denier (32.7-76.5 cN/tex) and a yarn tube compression insufficient to crush the tube core on which the yarn package is wound.
A limitation that is observed in the nylon yarn manufacturing process described by U.S. Pat. No. 5,750,215 are operating constraints which affect the extent to which the tension can be reduced between the draw zone and the relaxation zone. If the tension is reduced to too low of a level, the yarn becomes completely unstable leading to filamentation (or splaying of the individual filaments) and threadline breaks. The point at which this tension let-down becomes great enough to induce threadline instability is a relaxation ratio, according to Formula 1, greater than about 9%.Relaxation Ratio (%)=((RD−RR)/RD)×100, where  [1]                RD is the peripheral speed of the final stage draw rolls, and        RR is the peripheral speed of the relaxation rolls        
For many high strength fabric applications, the high shrinkages inherent to the high strength yarns used for such applications translate into high fabric shrinkages. For airbag applications, fabrics are required to exhibit both high strength, with a particular emphasis on the ability of the fabric to resist tearing and bursting when deployed, and low air permeability. Yarns that are suitable for airbag fabrics typically exhibit tenacities in the range of 60-85 cN/tex and hot air shrinkages (at 177° C. measured according to ASTM D 4974) of 5-15%. Low permeability can be achieved by applying a low permeability coating to at least one side of the fabric, or by producing a fabric with a very tight weave, or by some combination of those two measures. High strength is an essential characteristic of a fabric intended for this use since an airbag must be able to withstand the initial shock of an explosive inflation and, immediately thereafter, the impact of a passenger thrown against it. It must withstand these forces without bursting, tearing or appreciable stretching.
In most cases fabrics must be scoured to remove finish oils applied during yarn spinning and lubricants or bonding coatings applied prior to the weaving process. Thus, the woven fabrics are typically subjected to a washing step, followed by heating in dry air. The high shrinkage exhibited by the fabric in response to the washing and drying steps are used to advantage in order achieve a tighter weave and correspondingly lower air permeability. U.S. Pat. No. 5,581,856 teaches the manufacture of a fabric comprised of polyamide yarns having a hot air shrinkage at 160° C. of 6-15% (according to ASTM D4974). The as-woven fabric is subsequently subjected to treatment in an aqueous bath in a temperature range from 60° to 140° C. These conditions result in shrinkage leading to a further increase in density of the fabric which was already densely woven. The advantageous result is substantial closure of the pores of the fabric and a consequent improved resistance to gas permeability. In alternate processing for fabrics which require additional coating for either thermal protection or essentially zero air permeability, it is normal for the fabric to be “heat set” after washing. In this process the washed fabric is dried at temperatures close to or above those that will be experienced in coating and are typically in the region of 170° C.-225° C. Minimizing the degree of inherent shrinkage in the yarn allows drying at temperatures towards the lower end of this range and minimizes the risk of thermal damage to the yarn, an effect which usually manifests itself in the form of fabric discoloration.
“Air permeability” refers to the rate of air flow through a material and can be further defined as either “static air permeability” at a constant differential pressure across the fabric, or “dynamic air permeability” measured subsequent to a volume of air being introduced into a confined space over the fabric so as to generate an initial differential pressure. For the purpose of discussion throughout this application, air permeability will be of the static type which is defined as the volume rate of air at a differential pressure of 500 Pa through an area of 100 cm2 and expressed in I/dm2/min. This performance parameter is measured according to ISO 9237.
Fabrics intended for use in vehicle airbags have been woven by a variety of conventional weaving methods, including rapier, projectile, air-jet and water-jet weaving. Historically, many such fabrics have been formed using conventional rapier weaving machines wherein the weft yarn is drawn mechanically across the warp. Such weaving practices have been successful in producing the high weave density which is required for fabric that must exhibit low air permeability and which demonstrates the structural stability to withstand the inflation and collision forces when the airbag is deployed during an accident. However, rapier weaving machines can be significantly slower than alternative technologies such as water-jet weaving and can also inflict damage to the yarns during weaving due to frictional forces between the yarn and the weaving machine parts, as well as between the warp and weft yarns.
In water-jet weaving, the weft yarn is drawn through the shed of the warp yarns by means of a stream of water. This weaving method represents a much faster method of weft yarn insertion. Water-jet weaving can eliminate the need both for application of sizing compounds to the yarn and a separate washing or scouring operation. However, waterjet weaving historically has provided lower density weave constructions then rapier machines. In order to compensate, yarns having high breaking tenacities are often used so as to provide improved strength in the final fabric despite the less dense weave construction attainable by water-jet weaving. U.S. Pat. No. 5,421,378, incorporated herein by reference, has disclosed a method for manufacturing airbag fabrics by water-jet weaving of unsized yarns that is able to achieve weave densities comparable to rapier weaving.
While high fabric shrinkage may be used to advantage in order to achieve higher weave densities and low air permeabilities, it can also lead to manufacturing inefficiencies. In the production of one piece woven side-curtain airbag fabric, for example, the manufacturer has a desire to maximize the number of airbags that can be cut from one piece of fabric. The higher the shrinkage, the more constrained the manufacturer is in the number of pieces that can be cut from an as-woven fabric blank of a given width.
Side-curtain airbags are generally rectangular in shape and can, therefore, be made in contiguous rows across the width of the loom. Both sides of the inflatable structure may be cut as a one piece unit, which is subsequently folded in half to form an inflatable airbag. Alternatively, as in the case of jacquard looms, each such airbag can be made in one integral piece. The width of the fabric is limited first by the available width of weaving looms and second by the manageable complexity of jacquard heads. It is uncommon to find devices capable of weaving fabric more than 2.9 m wide. The fabric must then be shrunk to dimensionally stabilize it and, in the heretofore state-of-the-art case, shrinkages of the order of 8% are common. Hence, the airbag manufacturer is constrained in the minimum waste case to make an integral number of side curtain airbags across a width of (2.9-8%) m or 2.67 m. Thus, 3 airbags each of 0.89 m wide are optimal, or 4 each of 0.668 m or 5 each of 0.534 m or 6 each of 0.445 m and so forth.
Side-curtain airbags are required to fill the gap between the roof line of an automobile and the bottom of the window in the door, and this distance is rarely less than 0.4 m or more than 0.6 m. It is preferred that the shrinkage of the fabric in the weft direction is minimized to allow the maximum number of airbags to be manufactured.
Side-curtain airbags are engineered to remain inflated for a relatively longer period of time to protect a passenger against multiple and repetitive impacts within the automobile for the duration of an event in which the vehicles rolls over multiple times. Unlike front end collisions, in which the front end automobile occupant benefits both from the large energy-absorbing crumple zone and the front airbag, in side collisions there is no significant protection secondary to the side curtains and side airbags. As a consequence, side-curtain airbags are designed to operate with high internal pressures to maintain separation between the occupant and penetrating hazard, and to operate at a relatively high state of tension along their length to retain the occupant within the vehicle. It is required that these conditions are attained early in the inflation process and retained throughout a long duration rollover event. Thus, the short time allowed for the curtain to be positioned in the event of a crash leads to high inertial and pressure loading combined with axial tension which makes high strength yarn that much more important.
The technical requirements for side-curtain airbags underscore the need for high quality yarns with a shrinkage of less than 5% measured in air at 177° C. and with a tenacity equal to or greater than 80 cN/tex with a quality level appropriate for use in airbags or similar fabrics.
In view of the related art disclosures for preparing and realizing high tenacity polyamide yarns and fabrics made from such yarns, and further given the manufacturing inefficiencies encountered in the production of such high tenacity fabrics made from yarns that are not typically characterized by low shrinkage, it would be advantageous and desirable to identify improved procedures for efficiently producing multifilament polyamide yarns having tenacities equal to or greater than 80 CN/tex and hot air shrinkages (according to ASTM D 4974) less than 5%. Such fabrics would be especially desirable for industrial uses including airbags.